The present invention relates to a drop-in sensor comprising a bath thermocouple and a reverse-filling uniform geometry solidification chamber. The solidification chamber has no restrictions at the fill opening and a minimum dimension from the solidification chamber fill opening to the solidification chamber thermocouple measurement junction. The drop-in sensor according to the present invention allows for rapid and accurate measurements of the liquidus temperatures of molten metals.
During the steelmaking process, oxygen is combined with carbon dissolved in the steel to remove the carbon, while at the same time providing heat from the combustion process. When the carbon content, as well as the molten metal temperature, are known, the furnace operator can decide the extent of additional refining or end the process. It is therefore advantageous to have available information regarding both the temperature of the liquid metal and the carbon content.
U.S. Pat. No. 3,559,452 describes the usefulness and terminology of carbon determination in molten steel during the converter process. It is a common practice to employ disposable immersion sensors, such as those disclosed in U.S. Pat. No. 5,577,841, which are attached to a metal support lance and manually manipulated by an operator to obtain temperature and thermal analysis information. In many instances, ready access to the interior of steel processing vessels in order to contact the molten metal with disposable immersion measuring sensors, and thus determine the temperature and carbon content of the metal, is severely limited.
Large mechanical auxiliary lances, such as those described in U.S. Pat. No. 4,141,249, which are suitable for use in a converter process, have been constructed to immerse multifunctional sensor probes, such as those described in U.S. Patent Application Publication No. 2013/0098173, specially designed to attach to these auxiliary lances. However, there is a large capital expense required to modify an existing converter with an auxiliary lance. Once installed, the on-going maintenance of this equipment is difficult in the industrial environment of a typical steel mill, thereby increasing the cost per measurement. All of these expenses present a barrier to low cost steel production.
Drop-in or thrown-in devices have been developed to avoid the large capital investment of the auxiliary lance. Such measuring devices are dropped into the vessel from an access port well above the molten metal surface. These devices trail a signal cable that relays the thermocouple output to remote instrumentation providing data about the degree of processing accomplished, thereby enabling an operator to judge the extent of further processing required. Such devices for temperature measurement or thermal analysis are described in U.S. Pat. No. 3,357,598 and U.S. Pat. No. 3,463,005.
One difficulty in obtaining consistent and reliable molten metal sensor information from conventional drop-in devices is ensuring that the dropped sensor enters the liquid steel and remains submerged for a duration sufficient to obtain the desired measurements. Specifically, during the refining process, a large amount of slag floats on top of the molten steel and impedes the penetration of the dropped device through the slag layer and into the molten metal. Generally, an object dropped from a height above a liquid body will continue to be submerged below the liquid surface until its terminal velocity approaches zero, its buoyancy force counters the immersion force and the body reverses the trajectory towards floatation. More particularly, if the displaced volume of the molten steel is significantly greater than the weight of the portion of the drop-in sensor designed to be immersed therein, the sensor will float rather than become immersed. The buoyant force acting on the body immersed in the molten steel will act in an upward direction, thereby resulting in a floating position partially or totally above the liquid steel.
It has been traditionally recognized that the random drop of these sensors could, in some circumstances, result in errors when the thermocouple or an opening to a thermal arrest chamber is not orientated for optimal contact with the molten steel. Both a side-extending thermocouple, such as disclosed in U.S. Pat. No. 3,574,598, or a side-opening thermal arrest chamber, such as disclosed in U.S. Pat. No. 3,463,005, could face away from the metal and thus experience errors due to limited molten metal contact, partial filling or not being filled at all as a result of floatation.
To overcome this problem, improvements in these devices typically relied upon the bulk density of the device in relation to that of the liquid slag and that of the liquid steel, so as to aid in the descent of the device through the slag and into the steel, while providing some orientation toward the desired immersion direction. Bulk density is used herein to mean the overall density, including the sensor components and any voids within the overall immersed device and signal cable according to it submersed length. The multiple cross-sections in each configuration shown in U.S. Pat. No. 9,116,054 are graduated to accomplish a preferred orientation. That is, the total bulk density can be less than 7 g/cm3 as long as the a dense section of the measuring head has an apparent density greater than 7 g/cm3 necessary to ensure immersion of the measuring elements. Since liquid steel is approximately 90 percent as dense as solid steel, massive void free sections forward of the sensing elements are required to provide this orientation. The measuring elements are thus positioned at a distance removed from the forward face of the sensor. However, a sensing element at the forward face is the exact location that is favored for the longest exposure to the steel to be measured.
Still, at times, when the orientation is correct, that is, placing the temperature sensor in the metal and with a thermal arrest chamber that has filled with liquid metal, there exists a potential that accurate and reliable measurements are not obtained. U.S. Pat. No. 5,033,320 describes the necessary relation of the mass of the cooling chamber to the rate of solidification to obtain accurate thermal arrest information. While the bath temperature can be detected in a relatively short time, a longer time is required for the solidifying metal to reach a stable thermal balance between the released latent heat of fusion and the heat extraction of the cooling mass, and thus to obtain a liquidus plateau. From the detection of the temperature of a stable liquidus plateau, an accurate estimation of the carbon content can be made.
From the time the drop-in sensor is released, the trailing signal cable is constantly combusting. There is a potential for the destruction of the trailing signal cable to advance to a failed state before obtaining a longer duration liquidus plateau, such as that experienced for a high bath temperature, and thus failing to obtain the desired measurement. This is not new to the art. Various isolative and protective strategies have been employed in prior devices to delay the destruction of the signal cable by surrounding a portion of the signal cable exiting the measuring head with tubes of refractory material or cardboard, such as in U.S. Pat. No. 3,374,122
It is suggested by U.S. Pat. No. 4,881,824 that when the measurement head has an adjustable low density tube float on its end facing away from the immersion end and the signal cable is guided through this tube, such an arrangement provides for sufficient protection of the signal cable to obtain the desired measurements. At the same time, the low density material also serves as a counterbalance, orientating the measuring head and stabilizing it in a down-facing position. Although the float should orient the measuring device in a sensor element downward position, this is only true in a mono-density fluid. In the typical steel making vessel, where the slag typically has a density of approximately 3 g/cm3 and the steel typically has density of nearly 7 g/cm3, float-equipped devices have an overall bulk density that is much less than that of the liquid steel yet greater than that of the slag. Any additional buoyant material may orient the sensor within the first, less dense liquid layer, but no quantity of float will force a lower density measuring head to submerse itself below the second, higher density liquid.
Some known prior art drop-in devices include a rear-filling solidification chamber (i.e., the opening to the thermal arrest sample chamber is opposite to the immersion end), such as the device of Chinese Patent No. 201041556 (shown in FIG. 2). The chamber 2′ is constructed of resin sand and is axially located. However, because the chamber 2′ contains molten metal in contact with its inner wall and it outer wall is directly exposed to the molten bath, the chamber 2′ has an inefficient thermal capacity to chill the metal that has entered the inlet 11′. Without complete thermal isolation of the sample chamber 2′, the molten metal bath may act as a heat pump, thereby heating the liquid phase of the solidifying mass in the chamber 2′ and resulting in liquidus measurement errors which increase in number and magnitude as the molten metal superheat increases. In the conventional sense, superheat is determined by the difference between the temperature of the molten metal at the time of sampling and the thermal arrest temperature detected during its solidification. Accordingly, when a large portion of the sample chamber 2′ exists outside of the main measurement head 1′ and is in thermal communication with the molten metal bath, as shown in the prior art device of FIG. 2, it is subject to direct heating from the molten metal bath, thus diminishing the cooling ability of the sample chamber 2′ materials. Also, the exit of the signal cable 6 is secured to the measurement head 1 by a metal ring 5′, but is not restricted in its lateral movements, such that the sample chamber 2′ may be destroyed in certain orientations.
In some prior art drop-in probes, the solidification or sample chamber inflow/inlet opening is typically restricted and surrounded by a chilling mass which chills the inlet opening before the onset of solidification of the chamber contents, thus separating the potential of heat flux from the hotter surrounding liquid metal from delaying chilling or, at worst, reheating the remaining liquid of the two-phase solidifying mass.
Incremental improvements to drop-in measuring probes have eliminated many expected failure modes. However, many of these techniques are suitable only for drop-in devices when the time of the desired measurement occurs at the end of the converter process. This is disadvantageous, because a measurement of the temperature and carbon content is most desired prior to the end of the process, namely during the blowing process. It is at this time that adjustments to the process could be effected based on knowledge of the in-blow temperature and carbon content. There are numerous conditions at an intermediate measurement time, that is during blowing, which escalate the difficultly of a drop-in sensor penetrating and passing into the metal at a suitable depth in order to acquire reliable measurements. With the exception of the few previously mentioned devices, most of the known prior art is absent of improvements which address the difficultly of using drop-in sensors to obtain “in-blow” measurements.
Toward the end of decarburization, the amount of iron oxide transferred to the slag increases and thus reduces the viscosity, allowing for less drag, a reduction in slag volume due to the release of emulsified gases, and a leveling of the liquids at the end of blowing. All of these benefits are not available to those sensors dropped at in-blow. A large volume of slag that the sensor must pass through and a more viscous slag tend to delay the exposure of the thermocouple to the steel and delay the opening of the solidification chamber. Previously employed floats can impede that passage of the measuring head through the slag. Delays in passing through the slag promote freezing of the slag to the sensor's metal components, and thus increase the apparent volume and reduces bulk density. Retention of the desired submerged position is highly compromised.
An objective of the present invention is to provide a cost efficient drop-in sensor for use in a steelmaking vessel during and prior to the end of the blowing process for providing “in-blow” measurements. More particularly, in order to correct the deficiencies of prior art devices and extend the usefulness of drop-in sensors to “in-blow” measurements, the present invention employs a design based upon the kinetic potential of a weighted falling body to penetrate the liquid layers with an adequate submersion momentum, so as to remain below the slag metal interface for a critical duration. A measurement head according to an embodiment of the present invention has an in-situ density which is achieved by rapid filling of the solidification chamber during the initial descent into the liquid metal. Preferably, the rapid filling eliminates the initial void, and thus retards the normal buoyance-driven time to surface to measuring head. Once the liquidus chamber has been filled, the liquidus arrest plateau may occur even while the measurement head has surfaced, while providing a necessary time interval to obtain the required temperature measurement before floatation.